This invention pertains to apparatus and methods used in the evaluation of semiconducting films prior to device fabrication, and in particular to an apparatus and a method of evaluation based on semiconductor film efficiency, which is predictive of device efficiency.
Photovoltaic quantum efficiency, as a function of wavelength of incident light, QE(.lambda.), provides a measure of the number of photogenerated carriers formed and collected in a semiconducting layer when exposed to illumination. QE(.lambda.) is the ratio of the number of current carriers (e.g. electrons) passing through an external measurement circuit and the number of photons, at wavelength .lambda., incident on the device. The knowledge of photovoltaic quantum efficiency, along with the easily measured V.sub.oc, the open circuit voltage or the photovoltage across the cell at open circuit, allows one to estimate the maximum conversion efficiency (.eta.) of the solar cell. The term "collection length," 1.sub.c, signifies the thickness of the semiconducting layer over which photogenerated carriers can be collected; beyond this length the carriers recombine and are lost, thereby decreasing the maximum conversion efficiency of the solar cell. The knowledge of both quantities QE(.lambda.) and 1.sub.c is of great importance if discovered prior to fabricating solar cells, because together they determine the limit of ultimate conversion efficiency. Without this knowledge, the development of efficient solar cells becomes much more costly.
Current competitive design techniques for semiconductor device constructions employing new semiconducting film materials require a preliminary estimation of device performance without incurring the time or expense of complete prototype device construction. Prior art design techniques are limited in that two major parameters for measuring device efficiency, photovoltaic quantum efficiency QE(.lambda.) and collection length 1.sub.c, had to be determined from completed prototype photovoltaic or solar cells, and the prototypes had to be capable of generating a relatively high level, minimum, built-in photovoltage. Examples of such prototypes include completed p-n or p-i-n junctions, wherein p, n, and i, signify p-type, n-type and intrinsic-type conductivity, respectively. Other prototype examples include a Schottky barrier diode, consisting of a metal-semiconductor junction. In each of the above-described prototypes, two electrical contacts are necessary, with the second electrical contact being of the ohmic type. In all cases however, it was necessary for the solar cell to generate considerable built-in photovoltage which is the driving force that enables the photocurrent to be collected and measured. Two requirements must be met in order that a photovoltaic device produce the power required for performance evaluation. First, the sample device must contain material which absorbs light (the amorphous silicon i-layer) thereby creating electron-hole pairs in the material. Next, the device to be tested must have a "built-in" potential through which the collected carriers drop and thereby produce useful power. In conventional single crystal silicon solar cells, the carriers move to the region of potential drop (p-n junction) by diffusion. However in amorphous silicon solar cells this built-in potential also provides an internal field in the i-layer which aids in the collection of the carriers. An evaluation of more simple crystal samples that is independent of crystal type, and therefore independent of "built-in" potential, would be quite useful especially if the sample device by itself does not generate a photocurrent or photovoltage when illuminated i.e. when it does not exhibit a "built-in" potential. Such evaluation, if available, would allow an examination of the current collection properties of the i-layer in a sample test structure without the requirement of a built-in potential.
Prior evaluation methods suffer from the selective optical absorption of other preceding layers of the solar cell which further complicate the photovoltaic quantum efficiency determination and interpretation.
Further, it would be helpful if an evaluation method could provide information about the nature of the interface barriers. For example, the existence of a barrier gives rise to a built-in voltage at the interface which results in a depletion region inside the i-layer. In this region the electric field is high and the photogenerated carriers are swept out (collected as photocurrent). The width of the depletion region is also dependent on externally applied voltage. Thus, a careful study of QE(.lambda.) vs applied voltage allows one to determine the voltage barrier at the interface.
It is therefore an object of the present invention to provide a simplified apparatus and method for determining the quantum efficiency of semiconducting films without requiring the construction of an elaborate prototype sample.
It is another object of the present invention to provide a direct method of determining quantum efficiency of semiconductor samples which exhibit a heretofore unacceptably low level of "built-in" photovoltage.
It is an object of the present invention to provide an unambiguous interpretation of the blue response of a semiconducting sample i-layer such that any change in measured quantum efficiency is due only to the sample being tested.
Yet another object of the present invention is to provide a direct method of determining quantum efficiency of semiconducting films which avoids the selective optical absorption of illuminating radiation, through the elimination of preceding layers of a test prototype construction.
Another object of the present invention is to provide a test method which provides information about the nature of the interface barriers of a semiconductor sample.
A further object of the present invention is to provide a quantum efficiency test method which also provides an indication of the conductivity type of the sample being tested.
Yet another object of the present invention is to provide a universal quantum efficiency test method which is applicable to both types of conductivity materials, i.e., both n-type and p-type as well as intrinsic-type semiconducting materials.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.